Cathode ray tube (CRT) displays, such as those commonly used in desk-top computer screens, function as a result of a scanning electron beam from an electron gun, impinging on phosphors on a relatively distant screen. The electrons increase the energy level of the phosphors. When the phosphors return to their normal energy level, they release the energy from the electrons as a photon of light, which is transmitted through the glass screen of the display to the viewer.
Flat panel displays have become increasingly important in appliances requiring lightweight portable screens. Currently, such screens use electroluminescent or liquid crystal technology. A promising technology is the use of a matrix-addressable array of cold cathode emission devices to excite phosphor on a screen.
In U.S. Pat. No. 3,875,442, entitled "Display Panel," Wasa. et al. disclose a display panel comprising a transparent gas-tight envelope, two main planar electrodes which are arranged within the gas-tight envelope parallel with each other, and a cathodoluminescent panel. One of the two main electrodes is a cold cathode, and the other is a low potential anode, gate, or grid. The cathode luminescent panel may consist of a transparent glass plate, a transparent electrode formed on the transparent glass plate, and a phosphor layer coated on the transparent electrode. The phosphor layer is made of, for example, zinc oxide which can be excited with low energy electrons.
Spindt, et al. discuss field emission cathode structures in U.S. Pat. Nos. 3,665,241; and 3,755,704; and 3,812,559; and 4,874,981. To produce the desired field emission, a potential source is provided with its positive terminal connected to the gate, or grid, and its negative terminal connected to the emitter electrode (cathode conductor substrate). The potential source may be made variable for the purpose of controlling the electron emission current. Upon application of a potential between the electrodes, an electric field is established between the emitter tips and the low potential anode grid, thus causing electrons to be emitted from the cathode tips through the holes in the grid electrode. This structure is depicted in FIG. 1.
An array of points in registry with holes in low potential anode grids are adaptable to the production of cathodes subdivided into areas containing one or more tips, from which areas emissions can be drawn separately by the application of the appropriate potentials thereto.
The clarity, or resolution, of a field emission display is a function of a number of factors, including emitter tip sharpness, alignment and spacing of the gates (or grid openings) which surround the tips, pixel size, as well as cathode-to-gate and cathode-to-screen voltages. These factors are also interrelated. Another factor which effects image sharpness is the angle at which the emitted electrons strike the phosphors of the display screen.
The distance (d) that the emitted electrons must travel from the baseplate to the faceplate is typically on the order of several hundred microns. The contrast and brightness of the display are optimized when the emitted electrons impinge on the phosphors located on the cathodoluminescent screen, or faceplate, at a substantially 90.degree. angle.
Cold cathode field emission structures provide an electron beam which diverges, i.e., significantly in cross-sectional area, as the beam travels towards the anode. This is not desirable, particularly when the application is a high resolution flat panel display, since the "spot size" at the anode will limit the attainable resolution of the display.
The contrast and brightness of the display are not currently optimized due to the fact that the initial electron trajectories assume a substantially conical pattern having an apex angle of roughly 30.degree., which emanates from the emitter tip. Space-charge effects result in coulombic repulsion among emitted electrons which tends to further promote dispersion within the electron beam. However, the bulk of the beam spread in a conventional "tight gap" field emission display is a result of the initial lateral component imposed on the beam from the extraction grid, and not from coulombic self-repulsion of the beam itself. FIG. 1 illustrates the dispersion of an unfocused electron beam.
U.S. Pat. No. 5,070,282 entitled, "An Electron Source of the Field Emission Type," discloses a "controlling electrode" placed downstream of the "extracting electrode." U.S. Pat. No. 4,943,343 entitled, "Self-aligned Gate Process for Fabricating Field Emitter Arrays," discloses the use of photoresist in the formation of self-aligned gate structures.